Sierra Cosworth 4×4 Front Differential

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I cut up a sump from a Ford Sierra XR4x4 (It might have been a Cosworth??!?) to make a custom differential shaft housing. I added some strenghtening hollow tubes and two thick aluminium plates. The remants of the sump, were then TIG welded to the new material to form a very strong but light, bolt on assembly. The new set-up is actually lighter than the Sierra sump, yet I am positive that mine would be stronger.

Ford Sierra 4x4 Front Differential

Normally, the differential is bolted to the side of the sump and is free to move as the engine rocks around. Although the central drive shaft passes right underneath the crank and it’s housing is part of the sump, engine oil does pass inside. With my layout, the differential is in front of the engine, therefore a replacement central housing is needed.

I incorporated a sump reinforcing plate into the central assembly. This should hopefully strengthen the notoriously weak standard main differential housing. The whole assembly, including the sump plate, is now solidly mounted to the chassis in several places.

Now the differential is in, I fell much better about adding the missing triangulation from the front of the chassis. The differential assembly and the extra framework will work together to make this part of the chassis incredibly rigid. Before mounting the diff, when you hit the chassis with a hammer, it rang like a bell. Now it just makes a ‘tink’ sound like hitting a solid block. Extra tubes will also form mounts for the rocker arms for the inboard shock absorbers.

Because of the 19″ wheels I need to get the optimum differential ratio. The standard diff would give me a top speed of nearly 160mph. The Standard Cosworth and XR4x4 ratio is 3.62:1 but I have a 3.9:1 differential from a 2.0 DOHC GLS 4×4. This gives me a top speed around 140mph. The tag reads:

C9P5A CG02

Haynes Roadster 4x4 fron differential

OK, so not much in terms of actual work, but when there is ice on the workshop floor I’m quite pleased the progress. The only thing to go wrong was the discovery that the water pump outlet is a little too close to the prop-shaft UJ for comfort. The rubber hose would miss the UJ by around 7mm. Therefore, the water-pump will be the next thing to be cut up and modified. Not by much, in fact, I’ll cut a 5mm V of material out, bend the outlet up and re-weld the join. This should give me a much healthier 15-18mm clearance. I’ve also considered fitting a stabilizer bar to the top of the engine, to prevent it from twisting around too much.

You might ask why I went to all the effort of making a central section for the front diff. Other cars have been built with the nearside drive-shaft being a single long shaft attached directly to the differential. The central section is omitted.
If I did this then the extra space the long drive-shaft would need as it moves up and down would, cause it to foul the suspension push rods.
Some pick-ups and 4×4’s have axles that are potential candidates but are simply the wrong ratio and widths.
Another option would be to use two rear differentials, as they rotate correctly when reverse 360° (I checked). A special central shaft assembly would be needed but this is potentially the strongest setup.

The next jobs are to add the extra triangulation tubes and to mount the steering rack. I’ve always been a little worried about the rack as UJs are needed for the steering shaft to wrap its’ way around the diff. One UJ is giving me particular concerns. I won’t know just how tight it is going to be until the rack is solidly mounted.

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